minecraft 1.20.1, forge 47.3.0, zetter 1.20.1-0.21.5.

Playing in a multiplayer server, a friend and I will be painting, and while everything works perfectly on my end, for him, changes he makes to a painting aren't "synced" with me, or it seems the server, as when he disconnects and reconnects the changes made are reverted as if they hadn't happened (back to what it looks like for me). All the while I often cannot see changes made to a painting by him, while he can. Zetter works perfectly fine for him in a singleplayer world. He doesn't have connection issues or much lag at all to the server otherwise.

It's honestly quite difficult to pinpoint exactly what's going wrong here, if I have more information I can give I will. My best guess is just some kind of desynchronizing of what's happening on his client in the zetter gui vs what's happening on the server.

Generally speaking, when several players work together they sync not only "snapshots" but also their action history. This action history may have conflicts for actions made simultaneously, but differences should be fixed with snapshot syncs from time to time. If that does not happen and you have desynchronization permanently affecting paintings for some players, I would ask to do two things:
Check that the time is correct (btw are you in the same timezone? I don't think I had code accounting for timezone differences — maybe that is the problem).
If possible, take a recording of what is happening. Logs are useful too — they should throw some warnings when there are major unexpected time differences.